Underage volunteers will be sent into Sheffield shops over the summer looking for those selling alcohol to children.
South Yorkshire Police issued the warning following an operation carried out by the South East Sheffield Local Policing Team at the weekend, when 12 shops were visited by underage customers.
The force said one shop failed the test.
Inspector Jason Booth said: "Over the weekend we conducted an alcohol test purchase operation at 12 premises.
"Our underage volunteers were able to buy alcohol without adequate checks at one licensed premise.
"We will do supportive intervention work with that particularly shop and follow up checks will be conducted.
"We have more test purchase operations planned across the summer months."